The Effects of Sapphire Substrates Processes to The LED Efficiency

Recently, GaN-based blue and near-UV LEDs are extensively studied in the emerging solid-state lighting industry as a key technology. The light extraction efficiency (LEE) is very important factor in fabricating high brightness, high efficiency GaN-based light-emitting diode (LED). As well known, the large difference in the refractive index between the GaN(2.40) and air caused a narrow escape cone for the light in GaN crystal. One most used method is roughness or texture on interfaces, such as the patterned sapphire substrate (PSS) recently and the increasing efficiency of PSS LED is considered as a consequence of the light scattering by the PSS. Huang, etc developed the side wall shaping and truncated inverted pyramid (TIP) shaping technology to increase the external quantum efficiency (EQE) of GaN LED.

The flip-chip technology is also wildly studied because of its excellent performance in thermal conductivity and EQE these years. But the influence of sapphire substrate processes in the back-end process is few considered. In this work, the relation between extraction efficiency of GaN-based LED and several parameters of the sapphire substrate is studied, including thickness, roughness and package method. Firstly, the thickness of sapphire substrate in the back-end processes is studied. After choose a better sapphire thickness, we develop the roughness method using just a grinding process and the bottom face periodic pattern by dry etching technique to improve the external quantum efficiency. The different surface structure on the bottom surface of sapphire substrates are analyzed basis on geometry ray tracing simulation, and compared with experiments.


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